Page 393 - Industrial Ventilation Design Guidebook

348 CHAPTER S PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

where the mean level was 5 dB lower, the mean rating lay between "not at all
disturbing" and "somewhat disturbing." The difference in level resulted in a
clearly perceivable lowering of the average disturbance and inconvenience lev-
els. The fact that a 5-dB reduction in the ventilation noise level can result in such
a pronounced reduction of the perceived inconvenience can be explained by the
circumstance that a change in level in the low-frequency range has a signifi-
cantly greater effect on the loudness than would be the case in a high-frequency
sound range. Measures to achieve a reduction in ventilation noise of the order
of 5 dB can thus result in measurable gains in the form of lower inconvenience
reactions.
The range of answers to the question, "How does ventilation noise af-
fect your ability to perform your tasks?" reveals that about one in every five
office workers on average felt that ventilation noise made their work more
difficult. A significantly greater number assessed the higher level at 40 dB(A)
as an aggravating factor in the performance of their tasks at the office.
About 20% considered that the higher level made their work "somewhat"
or "much" more difficult. About 10% made a similar assessment at the
noise level of 35 dB(A).

5.4,5.2 Influence Due to Spectral Distribution
Systematic studies have been carried out for the purpose of studying
how low-frequency tones, broad-band components, and/or time fluctua-
4
tions in ventilation noise interfere with disturbance reactions, In one of
these studies, the respondents were exposed to ventilation noise that is rep-
resentative of the noise encountered in office premises. The respondents
were asked to use a rotating potentiometer to "set" the "most acceptable
noise level" and the "least acceptable noise level" for each noise, taking ac-
count of comfort, disturbance, and performance, while performing their
work at the same time. The noise level was maintained at a constant level of
40 dB{A).
When setting the most acceptable ventilation noise level consisting of a
single tone, all the respondents selected a lower tone frequency for both set-
tings than when they set the least acceptable level. The average frequencies set
for the most acceptable and the least acceptable noise levels were 58 and 380
Hz respectively. The disturbance experienced and the discomfort experienced
were significantly higher, and the performance was significantly lower, during
exposure to the least acceptable noise. A higher level of exertion was also ex-
perienced when exposed to the most acceptable noise.
The results clearly indicated that the ventilation noise was perceived as
most acceptable when the tone was situated in the lower part of the fre-
quency range. The experience of disturbance and the associated effects oc-
cur at exposure levels above the auditory perception threshold. Above this
level, the risk of these effects increases as the perceived loudness increases,
provided that the other conditions remain constant. Since the loudness can
be predicted relatively accurately by means of technical measurements, any
differences in the degree of disturbance can also be predicted by reference
to these measurements, provided that they are dependent on differences in
the loudness.